Warmer water temperature is another concern. The river lacks trees and plants to shade the water and concrete soaks up the sun's warmth, creating a habitat unsuitable for natives.

Drill said some of the new fish in the river have favorable characteristics. Mosquito fish help with pest control and carp make for good fishing.

"Part of (the river's) role is to provide low-income, underserved communities with a place to access nature, see native birds, and increase public health by having a safe area to walk and fish," Drill said. "I think there's value there too."